Emulsions and gels are used in a variety of applications, e.g. cosmetics, foods, insecticides and paints. Similarly, injectable pharmaceutical formulations are well known in the art. See Remington's Pharmaceutical Sciences, Mack Publishing Company, Easton, PA, 16th Edition, 1980. Usually such formulations are in the form of dispersions such as colloids, emulsions and suspensions. More recently, sustained release injectable formulations comprising polymers have been used.
Typical sustained release injectable formulations include viscous water-miscible vehicles such as gelatin, water-immiscible vehicles such as oils or oils thickened with such substances as aluminum monostearate, and thixotropic pellets (i.e. formulations which are viscous suspensions on standing, but which became fluid and syringeable when stirred or shaken vigorously, and then reform into viscous "pellets" after injection). Water-in-oil and more often oil-in-water emulsions are used in parenteral products, but usually not in sustained release formulations because such emulsions often present stability problems and may not provide long enough retention time of the active in the formulation, either because the dispersion breaks down or because the dispersion is too soluble in the surrounding body fluids (e.g. blood and lymph systems). Such stability problems are particularly associated with water soluble drugs. Protein drugs present the additional problem of enzymatic degradation in vivo. Practical problems also arise in preparing a formulation fluid enough to be syringeable, but viscous enough to remain in place one injected into the patient.